This invention relates to a process for retorting solid, hydrocarbon-containing material such as oil shale, coal and tar sand and dedusting and upgrading the effluent product stream.
Researchers have now renewed their efforts to find alternate sources of energy and hydrocarbons in view of recent rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal and tar sand by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah, and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot by recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons, can be done in surface retorts in aboveground vessels or in situ retorts underground. In principle, the retorting of shale and other hydrocarbon-containing materials such as coal and tar sand, comprise heating the solid hydrocarbon-containing material to an elevated temperature and recovering the vapors and liberated effluent. However, as medium grade oil shale yields approximately 25 gallons of oil per ton of shale, the expense of materials handling is critical to the economic feasibility of a commercial operation.
In order to obtain high thermal efficiency in retorting, carbonate decomposition should be minimized. Colorado Mahogany zone oil shale contains several carbonate minerals which decompose at or near the usual temperature attained when retorting oil shale. Typically, a 28 gallon per ton oil shale will contain about 23% dolomite (a calcium/magnesium carbonate) and about 16% calcite (calcium carbonate), or about 780 pounds of mixed carbonate minerals per ton. Dolomite requires about 500 BTU per pound and calcite about 700 BTU per pound for decomposition, a requirement that would consume about 8% of the combustible matter of the shale if these minerals were allowed to decompose during retorting. Saline sodium carbonate minerals also occur in the Green River formation in certain areas and at certain stratigraphic zones. The choice of a particular retorting method must therefore take into consideration carbonate decomposition as well as raw and spent materials handling expense, product yield and process requirements.
In surface retorting, oil shale is mined from the ground, brought to the surface, crushed and placed in vessels where it is contacted with a hot heat transfer carrier, such as ceramic or metal balls, hot spent shale or sand for heat transfer. The resulting high temperatures cause shale oil to be liberated from the oil shale leaving a retorted, inorganic material and carbonaceous material such as coke. The carbonaceous material can be burned by contact with oxygen at oxidation temperatures to recover heat and to form a spent oil shale relatively free of carbon. Spent oil shale which has been depleted in carbonaceous material is removed from the reactor and recycled as heat carrier material or discarded. The combustion gases are dedusted in a cyclone or electrostatic precipitator.
Some well-known processes of surface retorting are: N-T-U (Dundas Howes retort), Kiviter (Russian), Petrosix (Brazilian), Lurgi-Ruhrgas (German), Tosco II, Galoter (Russian), Paraho, Koppers-Totzek, Fushum (Manchuria), gas combustion and fluid bed. Process heat requirements for surface retorting processes may be supplied either directly or indirectly.
The Tosco II process and modifications thereof are described in U.S. Pat. Nos. 3,008,894, 3,034,979 and 3,058,903 and at pages 85-88 of the Synthetic Fuels Data Handbook by Cameron Engineers, Inc. (Second Edition 1978).
The Lurgi-Ruhrgas process and modifications thereof are described in U.S. Pat. Nos. 3,655,518; 3,703,442; 3,962,043; 4,038,045 and 4,054,492 and in the articles by Marnell, P., entitled Lurgi/Ruhrgas Shale Oil Process, published in Hydrocarbon Processing, pages 269-271 (September 1976); Schmalfeld, I. P., The Use of the Lurgi-Ruhrgas Process for the Distillation of Oil Shale, Volume 70, Number 3, Quarterly of the Colorado School of Mines, pages 129-145 (July 1975); Rammler, R. W.; The Retorting of Coal, Oil Shale, and Tar Sand by Means of Circulated Fine-Grained Heat Carriers as a Preliminary Stage in the Production of Synthetic Crude Oil, Volume 65, Number 4, Quarterly of the Colorado School of Mines, pages 141-167 (October 1970) and at pages 81-85 of the Synthetic Fuels Data Handbook by Cameron Engineers, Inc. (Second Edition 1978).
When retorting, thermocracking and coking are carried out simultaneously, carbonaceous residue, also referred to as "coke," "residual carbon" or "carbon residue," is deposited on retorted and heat carrier material and carried along with the effluent product stream. As the effluent product stream and retorted and heat carrier material circulate through various parts of the system at elevated temperatures, coke is deposited along the internal walls of pipes, vessels, cyclones and other equipment. The deposition and buildup of coke in the system restricts the flow rate and throughput capacity of the product stream, as well as the retort and heat carrier material. Coke accumulation also results in the formation of protrusions and depressions on the internal walls of pipes, vessels, cyclones and other equipment which interfere with the smooth linear flow of the effluent product stream as well as the retorted and heat carrier material. Coke deposits can further cause overheating and clog vital parts of equipment resulting in costly shutdown (downtime) and tedious removal of the accumulated carbonaceous residue. Moreover, the accumulation of coke in cyclones impedes dedusting of the effluent product stream. Delayed coking of crude oil and injection of steam to increase the velocity of crude oil and minimize coking are described at pages 131-136 and 145 of the book Petroleum Processing by R. J. Hengstebeck, published by McGraw-Hill Book Company, Inc. (1959).
During fluid bed, moving bed and other types of surface retorting, decrepitation of oil shale occurs creating a popcorning effect in which particles of oil shale collide with each other and impinge against the walls of the retort forming substantial quantities of minute entrained particulates of shale dust. The use of hot spent shale or sand as heat carrier material aggravates the dust problem. Rapid retorting is desirable to minimize thermal cracking of valuable condensable hydrocarbons, but increases the rate of decrepitation and amount of dust. Shale dust is also emitted and carried away with the effluent product stream during modified in situ retorting as a flame front passes through a fixed bed of rubblized shale, as well as in fixed bed surface retorting, but dust emission is not as aggravated as in other types of surface retorting.
Shale dust ranges in size from less than 1 micron to 1000 microns and is entrained and carried away with the effluent product stream. Because shale dust is so small, it cannot be effectively removed to commercially acceptable levels by conventional dedusting equipment.
The retorting, carbonization or gasification of coal, peat and lignite and the retorting or extraction of tar sand and gilsonite create similar dust problems.
After retorting, the effluent product stream of liberated hydrocarbons and entrained dust is withdrawn from the retort through overhead lines and subsequently conveyed to a separator, such as a single or multiple stage distillation column, quench tower, scrubbing cooler or condenser, where it is separated into fractions of light gases, light oils, middle oils and heavy oils with the bottom heavy oil fraction containing essentially all of the dust. As much as 40% by weight of the bottom heavy oil fraction consists of dust.
It is very desirable to upgrade the bottom heavy oil into more marketable products, such as light oils and middle oils, but because the heavy oil fraction is laden with dust, it is very viscous and cannot be pipelined. Dust laden heavy oil plugs up hydrotreaters and catalytic crackers, gums up valves, heat exchangers, outlet orifices, pumps and distillation towers, builds up insulative layers on heat exchange surfaces reducing their efficiency and fouls up other equipment. Furthermore, the dusty heavy oil corrodes turbine blades and creates emission problems. If used as a lubricant, dusty heavy oil is about as useful as sand. Moreover, the high nitrogen content in the dusty heavy oil cannot be refined with conventional equipment.
In an effort to solve this dust problem, electrostatic precipitators have been used as well as cyclones located both inside and outside the retort. Electrostatic precipitators and cyclones, however, must be operated at very high temperatures and the product stream must be maintained at or above the highest temperature attained during the retorting process to prevent any condensation and accumulation of dust on processing equipment. Maintaining the effluent steam at high temperatures is not only expensive from an energy standpoint, but it allows detrimental side reactions, such as cracking, coking and polymerization of the effluent product stream, which tends to decrease the yield and quality of condensable hydrocarbons.
Over the years various processes and equipment have been suggested to decrease the dust concentration in the heavy oil fraction and/or upgrade the heavy oil into more marketable light oils and medium oils. Such prior art dedusting processes and equipment have included the use of cyclones, electrostatic precipitators, pebble beds, scrubbers, filters, electric treaters, spiral tubes, ebullated bed catalytic hydrotreaters, desalters, autoclave settling zones, sedimentation, gravity settling, percolation, hydrocloning, magnetic separation, electrical precipitation, stripping and binding, as well as the use of diluents, solvents and chemical additives before centrifuging. Typifying those prior art processes and equipment and related processes and equipment are those found in U.S. Pat. Nos. 2,235,639; 2,717,865; 2,719,114; 2,723,951; 2,793,104; 2,879,224; 2,899,376; 2,904,499; 2,911,349; 2,952,620; 2,982,701; 2,968,603; 3,008,894; 3,034,979; 3,058,903; 3,252,886; 3,255,104; 3,468,789; 3,560,369; 3,684,699; 3,703,442; 3,784,462; 3,799,855; 3,808,120; 3,900,389; 3,901,791; 3,929,625; 3,974,073; 3,990,885; 4,028,222; 4,040,958; 4,049,540; 4,057,490; 4,069,133; 4,080,285; 4,088,567; 4,105,536; 4,151,073; 4,159,949; 4,162,965; 4,166,441; 4,182,672; 4,199,432; 4,220,522 and 4,246,093 as well as in the articles of Rammler, R. W., The Retorting of Coal, Oil Shale and Tar Sand By Means of Circulated Fine-Grained Heat Carriers as a Preliminary Stage in the Production of Synthetic Crude Oil, Volume 65, Number 4, Quarterly of the Colorado School of Mines, pages 141-167 (October 1970) and Schmalfeld, I. P., The Use of The Lurgi/Ruhrgas Process For The Distillation of Oil Shale, Volume 70, Number 3, Quarterly of the Colorado School of Mines, pages 129-145 (July 1975). These prior art processes and equipment have not been successful in decreasing the dust concentration in the heavy oil fraction to commercially acceptable levels.
It is therefore desirable to provide an improved process, which overcomes most, if not all, of the preceding problems.